1. Field of the Invention
The present invention relates to an exhaust emission control device for internal combustion engines, and a method of manufacturing the same, and more particularly to an exhaust emission control device which is capable of simultaneously combustion-removing toxic components, such as particulate substances (which will hereinafter be referred to as particulates), CO and hydrocarbon compounds (which will hereinafter be abbreviated to as HC), and which has a high durability, and a-method of manufacturing the same.
2. Description of the Prior Art
In recent years, particulates and NOx contained in exhaust gas discharged from internal combustion engines, especially, diesel engines have become an issue as toxic substances to the environment. Especially, particulates contain particles of an average grain size of not more than 1 .mu.m, and such particles float in the atmospheric air and are easily taken into human bodies by respiration. Moreover, it has been ascertained by recent clinical test results that these particles contain carcinogens. Therefore, the tightening of the control of the regulations concerning the discharge of these particulates is being discussed mainly in Japan, the U.S.A. and European countries.
Employment of DPF (Diesel Particulate Filter) as a particulate removing means which is adapted to remove particulates by filtering exhaust gas by using a heat-resistant porous body, such as a metal foam, a wire mesh, steel wool, a cell-sealed type ceramic honeycomb, a ceramic foam and open flow type ceramic honeycomb and metal honeycomb as a filter has been discussed. As particulates are collected by DPF, the pressure drop increases, so that a decrease in the engine output and deterioration of the fuel consumption occur. Therefore, the collected particulates are combustion-removed by a burner, an electric heater or a catalyst, thereby regenerating the filter.
The catalysts for burning particulates are roughly divided into noble-metal catalysts and base metal catalysts. A noble metal catalyst has a high durability and a high CO and HC removing performance but the particulate burning effect thereof has a problem. A base metal catalyst has a good particulate burning effect but it has a problem regarding its durability. Namely, the characteristics of the catalyst supported on a filter decrease due to SO.sub.2 in the exhaust gas.
The catalysts comprising a combination of a noble metal catalyst and a base metal catalyst include a catalyst comprising a combination of rhodium, copper, silver, iron, magnesium and zinc and disclosed in Japanese Patent Laid-Open No. 169534/1984, a catalyst comprising a combination of palladium and copper oxide and disclosed in Japanese Patent Laid-Open No. 64741/1993, and a catalyst comprising a metal oxide layer of oxides of Cu, Mn, Ni, Fe and Co and a catalyst metal such as platinum supported on the metal oxide layer, and disclosed in Japanese Patent Laid-Open No. 138026/1993.
Among these conventional catalysts, a catalyst having a practicality as well as a durability against SO.sub.2 -containing exhaust gas has not yet been found out. The exhaust gas temperature of a diesel engine is lower that of a gasoline engine, so that the former engine requires a low-temperature activity of the catalyst correspondingly to this. It has been found out that, when a conventional catalyst is used in a diesel engine exhaust emission control device, the low temperature activity is a problem.